The present invention relates to a high torque automatic stud driver. More particularly, the present invention relates to a stud driver having threaded jaws with thread relief provided on a leading edge of the jaws to reduce the likelihood of thread marking or thread deformation of the thread on the stud when the stud is driven under high torque conditions.
Automatic stud drivers are known in which a stud is rotated to thread or screw it into a workpiece. Such automatic stud drivers include a plurality of jaws which automatically clamp a stud once the stud is inserted in the driver, thread the stud into a workpiece, and then automatically release the stud without the requirement that the stud be unthreaded from the jaws of the driver. For example, see U.S. Pat. Nos. 4,513,643; 4,590,826; 4,476,749; 4,470,329 and 3,793,912. These known automatic stud drivers are most satisfactory under normal torque conditions i.e., torques that do not exceed the formula T=KDL where T is torque in inch pounds; K is the torque coefficient; D is the nominal stud diameter in inches; and L is the clamp load objective. Under normal torque conditions, the known stud drivers rotate the stud into the workpiece without damage to the threads on the portion of the stud clamped between the jaws.
However, under high torque conditions where the torque exceeds the torque in the aforementioned equation, the applicant discovered that damage or deformation occurs to the threaded portion of the stud clamped within the jaws. Such "thread marking" is undesirable from both an appearance standpoint and mechanical standpoint since thread markings disrupt the thread profile of the stud projecting from the workpiece, often to a degree wherein the thread becomes useless i.e., a subsequent item cannot be fully or easily threaded onto the portion of the stud projecting from the workpiece.
Accordingly, it is an object of the invention to provide an automatic stud driver which reduces the likelihood of thread marking under high torque conditions.
To achieve this and other objects, the present invention is directed to a stud driver in which the threads of the jaws are modified to adapt to high torque conditions. In particular, the applicant discovered that the thread markings occur on the stud at a portion corresponding to the location of the leading edge of the jaws when the stud is grasped by the jaws. Applicant discovered that the leading edge of the jaw tends to cut into the underside of the thread on the stud due to the small contact area and high stress. To relieve this stress, the present invention provides thread relief in a leading edge area of the jaw to create an increased contact area on the leading edge area of the jaws. Under high torque conditions, the shallower angle and increased contact area allow the leading edge of the jaw to approach the stud, without shearing or deforming the threads on the stud.
In a preferred embodiment, the automatic stud driver includes a plurality of stud gripping and rotationally driven jaws, each of the jaws having a leading edge area and a trailing edge area with reference to the rotational direction of the jaws. The plurality of jaws form a substantially cylindrical assembly having a central longitudinal axis. A transverse cross-section of each jaw defines an arc of a circle having a thread center point located along the central longitudinal axis. The thread center point also defines an intersection of first and second mutually orthogonal transverse axes, one of the transverse axes centrally dividing the arc into the leading edge area and trailing edge area.
Each of the jaws has an internal thread with a predetermined radius R1 originating from the thread center point. The leading edge area of each of the jaws has thread relief with a radius R2 originating from a thread relief center point located at a distance b from the thread center point in a direction away from the trailing edge of the jaw. In accordance with the preferred embodiment, the radius R2 is equal to about 75-150% of R1, and the distance b is equal to about 20-50% of the radius R2. In a preferred embodiment, the relief center point is located at a distance b from the thread center point in a direction away from the trailing edge of the jaw along a bisector of the first and second mutually orthogonal transverse axes.